This invention relates to the etching of aluminum electrolytic capacitor foil in an etching bath containing sodium chloride and boric acid or citrate ion source using direct current at 90.degree.-95.degree. C. to produce a foil that has a central metallic core and improved mechanical properties.
Etching of aluminum capacitor foil using direct current and a sodium chloride electrolyte bath is known. However, direct current etching has the disadvantage of mechanically weakening the foil because the tunnels formed by the etching penetrate deeply into the foil. Various remedies have been proposed, including alloying the aluminum with small amounts of other metals to make the foil more reactive to increase etch pit density and more easily etched without removing large amounts of aluminum which weakens the foil.
Still other processes use varying amounts of passivating agents or corrosion inhibitors to prevent dissolution of the foil surface while permitting deeper etching of the foil. Some of these agents have been found to have detrimental effects in subsequent processing of the foil.